1. Field of the Invention
The present invention generally relates to implantable medical devices, and more particularly to a multi-axis, multi-purpose sensor for detecting the patient's posture and activity level. The sensor can be used in various medical devices such as cardiac pacemakers, defibrillators, neural stimulators, drug dispensing pumps and the like.
2. Description of the Art
Motion detectors have been known and used in various technologies including home intrusion alarms and theft detectors. One such motion detector is marketed by Universal Photonix under the tradename Mr. Outside, for use as a single axis vehicle theft sensor. The sensor is designed to discriminate a rocking motion caused by wind from a deliberate attempt to hoist a vehicle for towing it away.
Universal Photonix' detector is based on the bubble cell technology used in aircraft attitude detection. The detector includes a cylindrically shaped cell of about one centimeter in diameter which is approximately half filled with an electrically conductive liquid fluid. Two curved outer electrodes and a central electrode are partly immersed in the fluid.
A voltage is impressed across the two outer electrodes and the voltage drop between the central and the outer electrodes is measured. If the cell were level, the voltage drop would be equally divided between the outer electrodes and the central electrode. However, if the cell were tilted, the voltage drop would be less between the central electrode and the outer electrode which is more immersed in the electrolyte. The sensor is only a single axis sensor and therefore cannot detect attitude changes not aligned with its sensitive axis.
This type of detector has not been utilized in medical applications. More specifically, the sensor has not been used as a dual purpose activity sensor and posture detector in cardiac pacemakers.
Posture and activity sensors are known in the medical field. One conventional medical posture or sensor is described in a patent application Ser. No. 07/490,065 (now U.S. Pat. No. 5,031,618), filed on Mar. 7, 1990, which is entitled "Position-Responsive Neuro Stimulator", and which is assigned to Medtronic, Inc.
This position sensor can be located in a chronically implanted programmable spinal cord stimulator, and determines whether the patient is in an erect or supine position. Whenever the patient reclines, the position sensor notifies the implanted spinal cord stimulator to continue stimulation at different preprogrammed parameters. This sensor serves various neurological functions. However, it does not address activity and multi-directional posture sensing.
One exemplary medical position sensor is described in U.S. Pat. No. 4,846,195 to Alt. Alt describes an implantable position and motion sensor which detects the physical orientation of the implanted medical device within the body. The sensor also indicates the state of rest or activity movement of the patient, and includes a chamber, a mercury ball confined within the chamber, and several electrodes for establishing contact with the ball to signify the physical orientation of the medical device.
The Alt sensor relies on the use of a mercury ball for the establishment of electrical contact. The toxicity of mercury renders the Alt sensor undesirable for certain applications, in particular for permanently implantable medical devices. Also, the sensor does not accurately reflect patient posture since it is not a true two-axis device and has rudimentary position determination.
Wherefore, it would be highly desirable to have a posture sensor for use in implantable medical devices. The sensor should be of a small size and should provide multi-directional and accurate readings of the patient's physical position. The sensor should also indicate the state of rest or activity movement of the patient, and should not use toxic and potential harmful material such as mercury.
Various activity sensors are known and used in the medical field. The following patents generally exemplify the technology in the art: U.S. Pat. No. 4,428,378 to Anderson; U.S. Pat. No. 4,896,068 to Nilsson; U.S. Pat. No. 4,869,251 to Lekholm et al; and European Patent Application No. 383,732 to Inguaggiato.
The Anderson activity sensor is mounted within the pacemaker and detects the general activity level of the patient for altering the escape interval in response to the detected activity level. The Nilsson sensor utilizes flexural type piezoelectric elements to detect the activity level. The Lekholm sensor has a hollow member with a freely movable member therein which generates a mechanical vibration upon movement within the hollow member. The Inguaggiato sensor includes a mass of mercury which can assume a shape determined by the gravitational force and which is variable as a result of forces applied to the mass due to movement.
These conventional activity sensors do not satisfactorily resolve the concerns associated with the posture sensors, and the need still exists for a multi-axis, multi-purpose sensor for simultaneously detecting the patient's position and activity level.